1. Field of the Invention
The present invention relates to a method of manufacturing an x-ray mask blank which is a material of an x-ray mask used as a mask in an x-ray lithography method and a method of manufacturing an x-ray membrane for the x-ray mask.
2. Description of the Related Art
In a semiconductor industry, as a technique for forming an integrated circuit constituted of a fine pattern on a silicon substrate or the like, a photolithography method for transferring the fine pattern by the use of a visible light and an ultraviolet light as an exposing electromagnetic wave is well known. However, a recent advance in a semiconductor technique greatly promotes a high integration of a semiconductor device such as VLSI, and this results in a requirement for the technique for transferring the fine pattern with high accuracy beyond a transfer limit (a principled limit due to a wavelength) of the visible light and the ultraviolet light for use in the conventional photolithography method. In order to transfer such a fine pattern, an x-ray lithography method using an x-ray whose wavelength is shorter than the wavelength of the visible light and the ultraviolet light is attempted.
FIG. 1 is a cross sectional view showing a structure of an x-ray mask for use in the x-ray lithography. FIG. 2 is a cross sectional view showing the structure of an example of an x-ray mask blank as an intermediate product obtained in an intermediate process during manufacturing the x-ray mask.
As shown in FIG. 1, an x-ray mask 1 comprises an x-ray membrane 12 for transmitting the x-ray and an x-ray absorbing film pattern 13a formed on the x-ray membrane 12. The x-ray membrane 12 is supported by a silicon frame body 11a which is formed by removing the other portion so that the periphery alone of the silicon substrate may remain. When this x-ray mask 1 is manufactured, the x-ray mask blank to be the intermediate product is manufactured in the intermediate process. This x-ray mask blank is further processed, so that the x-ray mask is obtained. In this industry, although, of course, the x-ray mask which is a finished product is to be dealt in, the x-ray mask blank which is the intermediate product is also often to be independently dealt in.
As shown in FIG. 2, an x-ray mask blank 2 comprises the x-ray membrane 12 formed on a silicon substrate 11 and an x-ray absorbing film 13 formed on the x-ray membrane.
Silicon nitride, silicon carbide, diamond or the like is generally used as the x-ray membrane 12. An amorphous material including tantalum (Ta) having an excellent resistance to x-ray radiation is often used as the x-ray absorbing film 13.
For the process of manufacturing the x-ray mask 1 from the x-ray mask blank 2, for example, the following method is used. That is, a resist film on which a desired pattern is formed is arranged on the x-ray mask blank 2 shown in FIG. 2. This pattern is then used as a mask so as to perform a dry etching, so that the x-ray absorbing film pattern is formed. After that, a center area formed on a rear surface and to be a window area of the x-ray membrane 12 is removed by a reactive ion etching (RIE) using 4-fluorocarbon (CF.sub.4) as etching gas. The remaining film (12a: see FIG. 1) is then used as the mask so as to etch the silicon by an etching liquid constituted of a mixed liquid of fluoric acid and nitric acid, whereby the x-ray mask 1 (see FIG. 1) is obtained. In this case, an electron beam (EB) resist is generally used as the resist, and the pattern is formed by means of an EB lithography.
For the x-ray membrane 12, a high transmittance to the x-ray, a high Young's modulus of elasticity, a proper tensile stress, a resistance to x-ray radiation, the high transmittance within a visible light range and the like are required. The characteristics will be described below. The transmittance to the x-ray is required during an exposure. The higher the transmittance is, the shorter a time required for the exposure can become. This is effective for improving throughput. The Young's modulus of elasticity has an influence on a strength of the film and a deformation of an absorber pattern. The higher the Young's modulus of elasticity is, the higher the film strength becomes. This is effective for suppressing misalignment. The proper tensile stress is needed in order that the film is self-supported. The resistance to x-ray radiation is required to cause no damage due to the x-ray radiation, because the x-ray membrane is irradiated with the x-ray during the exposure. As regards the transmittance within the visible light range, since an alignment of the mask attached to an x-ray stepper and a wafer is accomplished by the use of a light source within the visible light range, the high transmittance to an alignment light source is needed in order to achieve a highly accurate alignment. Furthermore, a film surface is required to be smooth. A surface smoothness is needed for a highly accurate pattern formation on the absorber.
In order to satisfy these requirements, various materials and manufacturing methods have been studied. Since it is confirmed that the silicon carbide has the highest Young's modulus of elasticity and causes no damage due to the x-ray in the silicon nitride, the silicon carbide (SiC) and the diamond which have been heretofore used as the x-ray membrane, it may safely be said that the silicon carbide is the most promising material. However, since the SiC film for the general use has a polycrystalline structure, the SiC film has the film surface which is rougher than 6 nm (Ra: a center-line average roughness) due to a crystalline structure. For smoothing of the surface of this SiC film, an etch back method and a mechanical polishing method are carried out after the film formation. The etch back method is the technique in which the rough SiC film is coated with the resist and the thus obtained smooth resist surface is transferred onto the SiC film by the dry etching. The mechanical polishing is the method in which a hard grain such as the diamond and alumina is used as an abrasive material so as to physically grind an unevenness on the surface of the SiC film. For example, according to Japanese Patent Publication No. 7-75219, the surface roughness of 20 nm or less is obtained by the etch back and the mechanical polishing. Although a definition of the surface roughness is not clear in this publication, this roughness is expected to be a maximum height (Rmax) and corresponds to about 2 nm or less in terms of Ra.
Recently, due to the advance in the photolithography technique, an introduction of the x-ray lithography has been performed later. At present, the introduction from a generation of 1 G bit-DRAM (design rule: 0.18 .mu.m) is anticipated. Even if the x-ray lithography is introduced from 1 G, the x-ray lithography is characterized by that it can be used through a plurality of generations up to 4 G, 16 G and 64 G. Assuming that the x-ray lithography is used for 64 G, a position precision required for the x-ray mask becomes severer, and the position precision is required to be as high as 10 nm. Furthermore, a mask pattern is required to have no defect regardless of a pattern size. Although the pattern defect can be corrected by a defect correcting unit, the number of practically correctable defects is limited to about 10 or less on the mask surface. A factor of the pattern defect is mainly caused due to the defect of the x-ray absorbing film, and more specifically, the important factor is the defect of the x-ray membrane. That is, if the defect (a contaminant or the like) is caused on the membrane, the defect is also inherited on the absorbing film formed on the membrane. Moreover, this faulty absorbing film causes the pattern defect after a mask processing. Therefore, it is necessary to exactly check the defect on the x-ray membrane and to process so that no defect or the least defect may be on the membrane. Furthermore, as regards the thin film such as an etching stop layer and a reflection preventing film formed as a sublayer of the x-ray absorbing film or the thin film such as an etching mask layer formed as a top layer of the x-ray absorbing film, since a presence of the defect on these thin films causes the pattern defect, it is necessary to check whether or not the defect is on these thin films. A minimum size of the defect affecting the pattern defect corresponds to a width of a minimum line of the pattern. Therefore, in case of the x-ray mask, it is necessary to exactly check the defect size of about 0.2 .mu.m. As a defect checking unit, the method of detecting a light scattering from the surface defect by the use of a laser light is generally used. For example, by Surfscan 6220 (KLA-Tencor), a minimum sensitivity of 0.09 .mu.m can be realized on a silicon wafer. However, in such a surface defect checking unit using the laser light, the detecting sensitivity to the defect is sensitively influenced by the surface roughness. Therefore, if the surface is rough, the roughness causes the light to be scattered, and thus the fine defect cannot be disadvantageously recognized (distinguished).